A three-jaw chuck for a lathe can be used for clamping a workpiece, particularly a crankshaft, on a clamping axis eccentric to the rotation axis of the clamping chuck.
It comprises a device for changing the spacing between the rotation axis and the clamping axis parallel thereto inside an axis spacing area along an axis spacing line extending radially through the rotation axis.
One of the clamping jaws with its clamping surface intersects the axis spacing plane going through the rotation axis on the axis spacing line and is adjustable so that the position of the clamping surface on the axis spacing line varies. The other clamp jaws are positioned on each side of the axis spacing plane slidably and adjustably in guides inclined symmetrically about the axis spacing plane and are jointly drivable.
This chuck allows crankshafts with substantially the same clamping diameter, but different crank strokes, to be machined by a lathe by changing the spacing between the rotation axis and the clamping axis.
A fit to another clamping diameter is also possible by replacing seat members in the clamp jaws.
To make this change of axis spacing, one known chuck has an axial chuck base attachable to the lathe spindle and a clamping head in whose clamp head body guided on the chuck base axially slidable on the axial spacing line all three clamp jaws are guided radially and slidably with respect to the clamping axis.
These clamp jaws are jointly slidable, e.g. by axially movable key rods driven by a drive piston slidable axially in the chuck base. They are jointly drivable centrally toward the clamping axis. Counterweights slidable toward the clamping head running oppositely provide a compensation for any unbalance occuring.
The chuck base, in principle, can be a chuck flange allowing connection to the lathe spindle in which the clamping head is guided axially slidable and is securable in its guide so that it is forced by a spacing piece determining its position by an adjusting spindle against the chuck base.
The clamp jaws and the key rods can be guided in the clamping head and can axially lock together. The key rods slidable in the adjusting direction are connected to one of them with the drive piston and are connected to clamping plates attached to the drive piston.
In the clamping head a workpiece opening, aligning the workpiece before clamping, can be provided with an aligning jaw pair whose movable aligning jaw likewise is guided in the clamping head.
Its own axially movable key rod operates this aligning jaw which, like the key rod for the clamp jaws, is slidably connected to a second clamping plate which is rigidly attached to a second drive piston provided to drive only the aligning jaw which is guided coaxial to the drive piston for the clamp jaws and in the latter toward the rotation axis of the chuck base.
As a result the clamping head up to the clamping plate and the drive piston forms a complete jaw chuck which is adjustable according to the crank stroke of the workpiece in the chuck base.
The counterweights for correcting imbalance are guided slidably in the clamping head. They act moreover to compensate as much as possible for the imbalance arising by sliding the clamping head in the chuck base. They slide in the clamping head opposite to its own sliding direction and are clamped in position by a spindle by a fitting spacing piece against the clamping head. Disadvantageously in such a clamping chuck the mass of the slidable clamping head is comparatively large so that the mass of the counterweights must be large to the same extent to compensate.
Also some residual imbalance always remains; such imbalance must be borne by the lathe spindle which must be made correspondingly large and requires massive and expensive spindle bearings.
The adjustment of the clamping head and the counterweights are problematical and expensive since these adjustments require their own working process. Moreover the clamping occuring after adjustment requires an additional working process to secure the adjustment which often does not remain without the adjustment reverting.
These disadvantages are reduced in chuck described in German Open Application (DE-OS) No. 34 46 687 in which the clamping head body is a slider slidable on the axis spacing line whose width transverse to the sliding direction is smaller than the diameter of the chuck base.
The counterweights guided and positioned in the chuck base on both sides of the clamping head body sliding opposite to the clamping head body for balancing are connected with each drive.
Both clamping jaws positioned inclined symmetrically relative to the axis spacing plane having a sliding direction opposite to the the clamping head body are divided into an inner jaw member and an outer jaw member. The inner jaw member in the clamping head body and the outer jaw member with the key rod engaged in it are guided in the chuck base and both jaw members are engaged with each other in a linear guide running parallel to the sliding direction of the clamping head body locked together by an undercut.
The clamp jaw slidable on the axial spacing line is formed in one piece without separation into inner and outer clamping members and is guided with only its key rods in the clamping head body. The key rod driving it is connected axially locked, but slidable in the sliding direction of the clamping head body to one of the key rods for the other clamp jaws is suspended with its clamping plate connected with its drive piston.
Also each of the aligning jaws drivable and slidable transverse to the axis spacing plane is separated into an inner jaw member guided inside the clamping head body and into an outer jaw member which is guided with its key rod and a drive piston operating the aligning jaws and which is engaged with the inner jaw member in a linear guide running parallel to the sliding direction of the clamping head body and locked in position by an undercut.
As a result no pieces of the clamping head forming the outer clamp jaws and the key rod belonging to it take part in this sliding and thus these elements do not contribute to the changes in the imbalance.
Outer and inner jaw members are coupled to each other but locked together in the sliding direction of the clamp jaws, e.g. dovetailed or in a T-groove guide, slidable in the linear guide connecting it with the clamping head so that the inner jaw member must participate in each sliding motion of the outer jaw member independently of the adjustment of the clamping head in the chuck base.
Thus the clamping head body containing only the adjustable clamping jaws adjustable in the axial spacing direction and the inner jaw member are formed so as to be small and light.
The counterweights for the imbalance which are correspondingly smaller and lighter in weight are located on both sides adjacent the clamping head body and can be guided and located in the chuck base. As a consequence, a light, compact, space saving structure results. Moreover this allows direct drive of the clamping head body with the counterweights so that the latter are adjusted automatically on changing the clamping head adjustment and thus automatically cause the compensation. On the other hand the chuck is complex and expensive.